Signature of King Edward VIIIThe 'R' and 'I' after his name indicate 'king' and 'emperor' in Latin ('Rex' and 'Imperator').

A king-emperor, the female equivalent being queen-empress, is a sovereign ruler who is simultaneously a king of one territory and emperor of another. This title usually results from a merger of a royal and imperial crown, but recognises that the two territories are different politically or culturally and in status (emperor being a higher rank than king). It also denotes a king's imperial status through the acquisition of an empire or vice versa.

The dual title signifies a sovereign's dual role, but may also be created to improve a ruler's prestige. Both cases, however, show that the merging of rule was not simply a case of annexation where one state is swallowed by another, but rather of unification and almost equal status, though in the case of the British monarchy the suggestion that an emperor is higher in rank than a king was avoided by creating the title "king-emperor" ("queen-empress") instead of "emperor-king" ("empress-queen").

The reigning King-Emperors or Queen-Empress used the initials R I (Rex Imperator or Regina Imperatrix) or the abbreviation Ind. Imp. (Indiae Imperator/Imperatrix) after their name (while the one reigning Queen-Empress, Victoria, used the initials R I, the three consorts of the married King-Emperors simply used R).

British coins, and those of the British Empire and Commonwealthdominions, routinely included some variation of the titles Rex Ind. Imp., although in India itself the coins said "Empress", and later "King Emperor." When, in August 1947, India became independent, all dies had to be changed to remove the latter two abbreviations, in some cases taking up to a year. In Great Britain, coins of George VI carried the title up to 1948.

The Serbian emperor Stefan Dušan (r. 1346–55), earlier king (1331–45), is attested with the title "Emperor of Greece and King of All Serb Lands and the Maritime" in a document dating to between 1347–56 (see also Emperor of the Serbs).[1] Dušan has been described as a "king-emperor".[2]